/*
 * Copyright 2002-2012 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.remoting.rmi;

import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;
import org.springframework.aop.support.AopUtils;
import org.springframework.lang.Nullable;
import org.springframework.remoting.RemoteConnectFailureException;
import org.springframework.remoting.RemoteInvocationFailureException;
import org.springframework.remoting.RemoteLookupFailureException;
import org.springframework.remoting.support.RemoteInvocation;
import org.springframework.remoting.support.RemoteInvocationBasedAccessor;
import org.springframework.remoting.support.RemoteInvocationUtils;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.net.URLStreamHandler;
import java.rmi.Naming;
import java.rmi.NotBoundException;
import java.rmi.Remote;
import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.rmi.server.RMIClientSocketFactory;

/**
 * {@link org.aopalliance.intercept.MethodInterceptor} for accessing conventional
 * RMI services or RMI invokers. The service URL must be a valid RMI URL
 * (e.g. "rmi://localhost:1099/myservice").
 *
 * <p>RMI invokers work at the RmiInvocationHandler level, needing only one stub for
 * any service. Service interfaces do not have to extend {@code java.rmi.Remote}
 * or throw {@code java.rmi.RemoteException}. Spring's unchecked
 * RemoteAccessException will be thrown on remote invocation failure.
 * Of course, in and out parameters have to be serializable.
 *
 * <p>With conventional RMI services, this invoker is typically used with the RMI
 * service interface. Alternatively, this invoker can also proxy a remote RMI service
 * with a matching non-RMI business interface, i.e. an interface that mirrors the RMI
 * service methods but does not declare RemoteExceptions. In the latter case,
 * RemoteExceptions thrown by the RMI stub will automatically get converted to
 * Spring's unchecked RemoteAccessException.
 *
 * @author Juergen Hoeller
 * @see RmiServiceExporter
 * @see RmiProxyFactoryBean
 * @see RmiInvocationHandler
 * @see org.springframework.remoting.RemoteAccessException
 * @see java.rmi.RemoteException
 * @see java.rmi.Remote
 * @since 29.09.2003
 */
public class RmiClientInterceptor extends RemoteInvocationBasedAccessor
        implements MethodInterceptor {

    private final Object stubMonitor = new Object();
    /**
     * spring ioc 容器初始化阶段是否需要远程查找
     */
    private boolean lookupStubOnStartup = true;
    /**
     * 是否还存
     */
    private boolean cacheStub = true;
    /**
     * 断线重连
     */
    private boolean refreshStubOnConnectFailure = false;
    private RMIClientSocketFactory registryClientSocketFactory;
    /**
     * 远程对象
     */
    private Remote cachedStub;

    /**
     * Set whether to look up the RMI stub on startup. Default is "true".
     * <p>Can be turned off to allow for late start of the RMI server.
     * In this case, the RMI stub will be fetched on first access.
     *
     * @see #setCacheStub
     */
    public void setLookupStubOnStartup(boolean lookupStubOnStartup) {
        this.lookupStubOnStartup = lookupStubOnStartup;
    }

    /**
     * Set whether to cache the RMI stub once it has been located.
     * Default is "true".
     * <p>Can be turned off to allow for hot restart of the RMI server.
     * In this case, the RMI stub will be fetched for each invocation.
     *
     * @see #setLookupStubOnStartup
     */
    public void setCacheStub(boolean cacheStub) {
        this.cacheStub = cacheStub;
    }

    /**
     * Set whether to refresh the RMI stub on connect failure.
     * Default is "false".
     * <p>Can be turned on to allow for hot restart of the RMI server.
     * If a cached RMI stub throws an RMI exception that indicates a
     * remote connect failure, a fresh proxy will be fetched and the
     * invocation will be retried.
     *
     * @see java.rmi.ConnectException
     * @see java.rmi.ConnectIOException
     * @see java.rmi.NoSuchObjectException
     */
    public void setRefreshStubOnConnectFailure(boolean refreshStubOnConnectFailure) {
        this.refreshStubOnConnectFailure = refreshStubOnConnectFailure;
    }

    /**
     * Set a custom RMI client socket factory to use for accessing the RMI registry.
     *
     * @see java.rmi.server.RMIClientSocketFactory
     * @see java.rmi.registry.LocateRegistry#getRegistry(String, int, RMIClientSocketFactory)
     */
    public void setRegistryClientSocketFactory(RMIClientSocketFactory registryClientSocketFactory) {
        this.registryClientSocketFactory = registryClientSocketFactory;
    }


    @Override
    public void afterPropertiesSet() {
        super.afterPropertiesSet();
        prepare();
    }

    /**
     * Fetches RMI stub on startup, if necessary.
     *
     * @throws RemoteLookupFailureException if RMI stub creation failed
     * @see #setLookupStubOnStartup
     * @see #lookupStub
     */
    public void prepare() throws RemoteLookupFailureException {
        // Cache RMI stub on initialization?

        if (this.lookupStubOnStartup) {
            // 获取remote对象
            Remote remoteObj = lookupStub();
            if (logger.isDebugEnabled()) {
                if (remoteObj instanceof RmiInvocationHandler) {
                    logger.debug("RMI stub [" + getServiceUrl() + "] is an RMI invoker");
                }
                else if (getServiceInterface() != null) {
                    // 是否接口
                    boolean isImpl = getServiceInterface().isInstance(remoteObj);
                    logger.debug("Using service interface [" + getServiceInterface().getName() +
                            "] for RMI stub [" + getServiceUrl() + "] - " +
                            (!isImpl ? "not " : "") + "directly implemented");
                }
            }
            if (this.cacheStub) {
                // 缓存 remote 对象
                this.cachedStub = remoteObj;
            }
        }
    }

    /**
     * Create the RMI stub, typically by looking it up.
     * <p>Called on interceptor initialization if "cacheStub" is "true";
     * else called for each invocation by {@link #getStub()}.
     * <p>The default implementation looks up the service URL via
     * {@code java.rmi.Naming}. This can be overridden in subclasses.
     *
     * @return the RMI stub to store in this interceptor
     * @throws RemoteLookupFailureException if RMI stub creation failed
     * @see #setCacheStub
     * @see java.rmi.Naming#lookup
     */
    protected Remote lookupStub() throws RemoteLookupFailureException {
        try {
            Remote stub = null;
            if (this.registryClientSocketFactory != null) {
                // RMIClientSocketFactory specified for registry access.
                // Unfortunately, due to RMI API limitations, this means
                // that we need to parse the RMI URL ourselves and perform
                // straight LocateRegistry.getRegistry/Registry.lookup calls.
                // 通过 serviceUrl 创建 URL
                URL url = new URL(null, getServiceUrl(), new DummyURLStreamHandler());
                // url 的协议
                String protocol = url.getProtocol();
                if (protocol != null && !"rmi".equals(protocol)) {
                    throw new MalformedURLException("Invalid URL scheme '" + protocol + "'");
                }
                // 获取host
                String host = url.getHost();
                // 获取port
                int port = url.getPort();
                // 获取serviceName
                String name = url.getPath();
                if (name != null && name.startsWith("/")) {
                    name = name.substring(1);
                }
                // 创建 Registry
                Registry registry = LocateRegistry.getRegistry(host, port, this.registryClientSocketFactory);
                // 获取Remote
                stub = registry.lookup(name);
            }
            else {
                // Can proceed with standard RMI lookup API...
                stub = Naming.lookup(getServiceUrl());
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Located RMI stub with URL [" + getServiceUrl() + "]");
            }
            return stub;
        }
        catch (MalformedURLException ex) {
            throw new RemoteLookupFailureException("Service URL [" + getServiceUrl() + "] is invalid", ex);
        }
        catch (NotBoundException ex) {
            throw new RemoteLookupFailureException(
                    "Could not find RMI service [" + getServiceUrl() + "] in RMI registry", ex);
        }
        catch (RemoteException ex) {
            throw new RemoteLookupFailureException("Lookup of RMI stub failed", ex);
        }
    }

    /**
     * Return the RMI stub to use. Called for each invocation.
     * <p>The default implementation returns the stub created on initialization,
     * if any. Else, it invokes {@link #lookupStub} to get a new stub for
     * each invocation. This can be overridden in subclasses, for example in
     * order to cache a stub for a given amount of time before recreating it,
     * or to test the stub whether it is still alive.
     *
     * @return the RMI stub to use for an invocation
     * @throws RemoteLookupFailureException if RMI stub creation failed
     * @see #lookupStub
     */
    protected Remote getStub() throws RemoteLookupFailureException {
        if (!this.cacheStub || (this.lookupStubOnStartup && !this.refreshStubOnConnectFailure)) {
            // 如果缓存stub存在直接获取,否则创建
            return (this.cachedStub != null ? this.cachedStub : lookupStub());
        }
        else {
            synchronized (this.stubMonitor) {
                if (this.cachedStub == null) {
                    this.cachedStub = lookupStub();
                }
                return this.cachedStub;
            }
        }
    }


    /**
     * Fetches an RMI stub and delegates to {@code doInvoke}.
     * If configured to refresh on connect failure, it will call
     * {@link #refreshAndRetry} on corresponding RMI exceptions.
     *
     * @see #getStub
     * @see #doInvoke(MethodInvocation, Remote)
     * @see #refreshAndRetry
     * @see java.rmi.ConnectException
     * @see java.rmi.ConnectIOException
     * @see java.rmi.NoSuchObjectException
     */
    @Override
    public Object invoke(MethodInvocation invocation) throws Throwable {
        // 获取remote
        Remote stub = getStub();
        try {
            // 真正的invoke调用
            return doInvoke(invocation, stub);
        }
        catch (RemoteConnectFailureException ex) {
            return handleRemoteConnectFailure(invocation, ex);
        }
        catch (RemoteException ex) {
            if (isConnectFailure(ex)) {
                return handleRemoteConnectFailure(invocation, ex);
            }
            else {
                throw ex;
            }
        }
    }

    /**
     * Determine whether the given RMI exception indicates a connect failure.
     * <p>The default implementation delegates to
     * {@link RmiClientInterceptorUtils#isConnectFailure}.
     *
     * @param ex the RMI exception to check
     * @return whether the exception should be treated as connect failure
     */
    protected boolean isConnectFailure(RemoteException ex) {
        return RmiClientInterceptorUtils.isConnectFailure(ex);
    }

    /**
     * Refresh the stub and retry the remote invocation if necessary.
     * <p>If not configured to refresh on connect failure, this method
     * simply rethrows the original exception.
     *
     * @param invocation the invocation that failed
     * @param ex         the exception raised on remote invocation
     * @return the result value of the new invocation, if succeeded
     * @throws Throwable an exception raised by the new invocation,
     *                   if it failed as well
     * @see #setRefreshStubOnConnectFailure
     * @see #doInvoke
     */
    @Nullable
    private Object handleRemoteConnectFailure(MethodInvocation invocation, Exception ex) throws Throwable {
        if (this.refreshStubOnConnectFailure) {
            String msg = "Could not connect to RMI service [" + getServiceUrl() + "] - retrying";
            if (logger.isDebugEnabled()) {
                logger.warn(msg, ex);
            }
            else if (logger.isWarnEnabled()) {
                logger.warn(msg);
            }
            return refreshAndRetry(invocation);
        }
        else {
            throw ex;
        }
    }

    /**
     * Refresh the RMI stub and retry the given invocation.
     * Called by invoke on connect failure.
     *
     * @param invocation the AOP method invocation
     * @return the invocation result, if any
     * @throws Throwable in case of invocation failure
     * @see #invoke
     */
    @Nullable
    protected Object refreshAndRetry(MethodInvocation invocation) throws Throwable {
        Remote freshStub = null;
        synchronized (this.stubMonitor) {
            this.cachedStub = null;
            freshStub = lookupStub();
            if (this.cacheStub) {
                this.cachedStub = freshStub;
            }
        }
        return doInvoke(invocation, freshStub);
    }

    /**
     * Perform the given invocation on the given RMI stub.
     *
     * @param invocation the AOP method invocation
     * @param stub       the RMI stub to invoke
     * @return the invocation result, if any
     * @throws Throwable in case of invocation failure
     */
    @Nullable
    protected Object doInvoke(MethodInvocation invocation, Remote stub) throws Throwable {
        if (stub instanceof RmiInvocationHandler) {
            // RMI invoker
            try {
                return doInvoke(invocation, (RmiInvocationHandler) stub);
            }
            catch (RemoteException ex) {
                throw RmiClientInterceptorUtils.convertRmiAccessException(
                        invocation.getMethod(), ex, isConnectFailure(ex), getServiceUrl());
            }
            catch (InvocationTargetException ex) {
                Throwable exToThrow = ex.getTargetException();
                RemoteInvocationUtils.fillInClientStackTraceIfPossible(exToThrow);
                throw exToThrow;
            }
            catch (Throwable ex) {
                throw new RemoteInvocationFailureException("Invocation of method [" + invocation.getMethod() +
                        "] failed in RMI service [" + getServiceUrl() + "]", ex);
            }
        }
        else {
            // traditional RMI stub
            try {
                // RMI客户端工具类执行
                return RmiClientInterceptorUtils.invokeRemoteMethod(invocation, stub);
            }
            catch (InvocationTargetException ex) {
                Throwable targetEx = ex.getTargetException();
                if (targetEx instanceof RemoteException) {
                    RemoteException rex = (RemoteException) targetEx;
                    throw RmiClientInterceptorUtils.convertRmiAccessException(
                            invocation.getMethod(), rex, isConnectFailure(rex), getServiceUrl());
                }
                else {
                    throw targetEx;
                }
            }
        }
    }

    /**
     * Apply the given AOP method invocation to the given {@link RmiInvocationHandler}.
     * <p>The default implementation delegates to {@link #createRemoteInvocation}.
     *
     * @param methodInvocation  the current AOP method invocation
     * @param invocationHandler the RmiInvocationHandler to apply the invocation to
     * @return the invocation result
     * @throws RemoteException           in case of communication errors
     * @throws NoSuchMethodException     if the method name could not be resolved
     * @throws IllegalAccessException    if the method could not be accessed
     * @throws InvocationTargetException if the method invocation resulted in an exception
     * @see org.springframework.remoting.support.RemoteInvocation
     */
    @Nullable
    protected Object doInvoke(MethodInvocation methodInvocation, RmiInvocationHandler invocationHandler)
            throws RemoteException, NoSuchMethodException, IllegalAccessException, InvocationTargetException {

        if (AopUtils.isToStringMethod(methodInvocation.getMethod())) {
            return "RMI invoker proxy for service URL [" + getServiceUrl() + "]";
        }

        /**
         * 1. 参数组装成对象{@link RemoteInvocationBasedAccessor#createRemoteInvocation(org.aopalliance.intercept.MethodInvocation)}
         * 2. invoke 执行 简单来说就是调用{@link RmiInvocationHandler#invoke(RemoteInvocation)}方法
         */
        return invocationHandler.invoke(createRemoteInvocation(methodInvocation));
    }


    /**
     * Dummy URLStreamHandler that's just specified to suppress the standard
     * {@code java.net.URL} URLStreamHandler lookup, to be able to
     * use the standard URL class for parsing "rmi:..." URLs.
     */
    private static class DummyURLStreamHandler extends URLStreamHandler {

        @Override
        protected URLConnection openConnection(URL url) throws IOException {
            throw new UnsupportedOperationException();
        }
    }

}
